1. Field of the Invention
The present invention relates to an oscillation signal processing apparatus, and more particularly, to an oscillation signal processing apparatus which obtains a tuner reference signal and a subcarrier by oscillating one quartz oscillator included in a television apparatus, a television apparatus using the oscillation signal processing apparatus, and an oscillation signal processing control method.
2. Description of the Prior Art
FIG. 6 is a block diagram showing the arrangement of principal elements of a conventional television apparatus having a circuit to oscillate one quartz oscillator to oscillate a tuner reference signal and a subcarrier to demodulate color difference signal. In FIG. 6, a tuner 1 is connected to a SAW filter 2 and a PLL circuit 5. A television broadcast signal is received via an antenna, and based on the received signal, an intermediate frequency signal is outputted. The tuner 1, having a so-called PLL tuning mechanism, obtains a desired local oscillation frequency with the output signal from the PLL circuit 5 as a reference.
The intermediate frequency signal outputted from the tuner 1 is inputted via the SAW filter 2 and a VIF circuit 3 into a signal processing circuit. Through correction by the SAW filter 2 then a predetermined amplification by the VIF circuit 3, the signal is inputted into a signal processing circuit 4. The signal processing circuit 4 detects an input video intermediate frequency signal and outputs RGB signals. Upon signal detection, a subcarrier to demodulate a color difference signal from a carrier chrominance signal is obtained from the PLL circuit 5 connected to the signal processing circuit 4. The signal processing circuit 4 also outputs a color burst signal as a reference signal for oscillation of the subcarrier to the PLL circuit 5.
The PLL circuit 5 is connected to a quartz oscillator 6, and feed-back controls an oscillation signal from the quartz oscillator 6 with the input color burst signal as a reference signal. As a result, an oscillation signal synchronized with the color burst signal is outputted, and the oscillation signal is used as a reference signal for the subcarrier inputted into the signal processing circuit 4 and for the tuner 1. That is, to obtain the reference signal in the tuner and the subcarrier in signal detection, any oscillation mechanism is necessary. In the oscillation signal processing apparatus of the conventional art, the common quartz oscillator 6 is used for obtaining both oscillation signals.
On the other hand, Japanese Published Unexamined Utility Model Registration No. Hei 3-53050 discloses a technique to obtain a subcarrier by inputting a color burst signal, based on a video intermediate frequency signal generated via a tuner and a VIF circuit, into a PLL circuit having an APC (Automatic Phase Control) filter and VCO (Voltage Controlled Oscillator). The time constant of the APC filter is variable. In this construction, the response speed of the PLL circuit is changed by changing the time constant of the APC filter in a just-tuning period and in a non-just-tuning period, thus preventing occurrence of tuning noise.
The conventional oscillation signal processing apparatuses have the following problems.
That is, in the former conventional art, the tuner 1 and the signal processing circuit 4 can be driven by only one quartz oscillator 6, however, in the PLL circuit 5, in a case where noise is inputted or an input signal is unstable, the phase, level and the like of the oscillation signal become unstable. The instability of oscillation signal from the PLL circuit 5 causes instability of the intermediate frequency signal outputted from the tuner 1, and further causes instability in the signal inputted into the PLL circuit 5, thus causing tuning error and tuning shift.
Further, in the technique disclosed in Japanese Published Unexamined Utility Model Registration No. Hei 3-53050, although the PLL circuit can be controlled in accordance with the operation state of the tuner, there is no solution for the problem of instability in tuning operation in use of construction as in the former art to drive the tuner and the signal processing circuit by one quartz oscillator.
The present invention has been made in consideration of the above problems, and has its object to provide an oscillation signal processing apparatus capable of stable tuning operation while obtaining a tuner reference signal and a subcarrier by oscillating one quartz oscillator, a television apparatus using the oscillation signal processing apparatus, and an oscillation signal processing control method.
The above object is attained by providing an oscillation signal processing apparatus for oscillating a subcarrier to demodulate a color difference signal from a carrier chrominance signal, comprising: PLL oscillation unit that performs predetermined oscillation synchronized with a color burst signal by a predetermined PLL and outputting an oscillation output as the subcarrier; external output unit that outputs the oscillation output from the PLL oscillation unit as a reference signal for an external tuner; and PLL oscillation control unit that controls the PLL oscillation unit to stop a phase comparison operation upon start of tuning operation by the external tuner, so as to stabilize the reference signal from the external output unit.
In the present invention having the above construction, the oscillation signal processing apparatus oscillates a subcarrier to demodulate a color difference signal from a carrier chrominance signal. For this purpose, the PLL oscillation unit performs predetermined oscillation synchronized with the color burst signal by a predetermined PLL, and outputs the oscillation signal as the subcarrier. When the PLL oscillation unit outputs the oscillation signal, the external output unit outputs the oscillation output from the PLL oscillation unit as the reference signal for the external tuner. The PLL oscillation control unit controls the PLL oscillation unit upon start of tuning operation by the external tuner to stop the phase comparison operation, to stabilize the reference signal outputted from the external output unit.
That is, the oscillation signal from the PLL oscillation unit is used as the reference signal for the tuner and as the subcarrier for demodulation of color difference signal. The tuner selects only a necessary signal from a received television broadcast signal and high-frequency amplifies the selected signal, converts signal into an intermediate frequency signal and outputs the signal, in accordance with various methods. For example, in a frequency synthesizer method, phase detection is made between a signal frequency-converted from a reference signal via a fixed frequency divider, and a signal frequency-converted from an oscillation signal oscillated from a local oscillator via a variable frequency divider for frequency division in correspondence with a user""s desired channel.
The frequency of the local oscillator is changed by a tuner PLL circuit which feed-back controls the local oscillator by using the result of phase detection, there by a desired local oscillation signal is obtained. Then, the local oscillation signal corresponding to the desired channel is mixed with a signal high-frequency amplified from a signal received via an antenna, into a 58.75 MHz intermediate frequency signal. Note that in the present invention, as the reference signal is the PLL oscillation signal outputted from the external output unit, if the phase and/or level of the reference signal is unstable, the reference for the tuner PLL becomes unstable, which causes instability in the output from the local oscillator, as a result, causing tuning error or tuning shift.
Accordingly, in the present invention, the output from the external output unit upon start of tuning operation is stabilized. For this purpose, the PLL oscillation unit does not perform the phase comparison operation. That is, as the PLL is used for comparing phases of the color burst signal as the reference signal and the predetermined oscillation signal to control the oscillation signal so as to eliminate a phase difference therebetween, if the comparison operation is not performed, the oscillation signal is not varied. Accordingly, a stable reference signal is used upon start of tuning operation, and tuning error can be prevented.
According to the present invention, as the tuner reference signal can be stabilized upon start of tuning operation, an oscillation signal processing apparatus capable of stable tuning operation while obtaining a tuner reference signal and a subcarrier by oscillating one quartz oscillator can be provided.
In a television apparatus using the oscillation signal processing apparatus, a stable tuning operation can be performed. Further, a similar advantage can be obtained by a similar oscillation signal processing control method other than the invention as an apparatus having a material substance.
Note that as a construction for oscillation by the PLL oscillation unit, a so-called oscillation circuit can be provided, however, it is more preferable to use a quartz oscillator which oscillates with a very stable natural oscillation regardless of variation of temperature, a power-source voltage or the like. Further, as the PLL oscillation unit, any unit can be used as long as it can output the oscillation from the quartz oscillator or the like in synchronization with a color burst signal. For example, the PLL oscillation unit may be constructed with a burst amplification circuit which performs predetermined amplification on an input intermediate frequency signal and outputs a color burst signal, a VCO circuit which oscillates the quartz oscillator in accordance with an input voltage, and an APC circuit which outputs a voltage corresponding to a phase difference between output signals from the burst amplification circuit and the VCO circuit. Further, the external output unit may be any unit as long as it outputs the oscillation output from the PLL oscillation unit as the external tuner reference signal. For example, to obtain a desired signal, the external output unit may output the PLL oscillation output to the tuner via a buffer.
Further, the PLL oscillation control unit may be any unit as long as it controls the PLL oscillation unit upon start of tuning operation by the external tuner to stop the phase comparison operation. As a particular example of this purpose, in another aspect of the present invention, the PLL oscillation unit has a burst amplification circuit to perform predetermined amplification prior to the phase comparison operation to reduce amplification variation of the color burst signal, and the PLL oscillation control unit has a switch to reduce the gain of the burst amplification circuit to a minimum.
In the aspect of the present invention, the PLL oscillation unit has the burst amplification circuit to perform predetermined amplification prior to the phase comparison operation so as to reduce the amplitude variation of the color burst signal. Further, the PLL oscillation control unit has the switch to reduce the gain of the burst amplification circuit to a minimum. As the gain is reduced to a minimum by the switch, a no-input signal state can be obtained. In this state, the phase comparison operation by the PLL is not performed, and oscillation is made in free-run frequency. Accordingly, a stable signal can be outputted to the PLL oscillation unit without influence by input noise or the like.
According to this aspect of the present invention, the phase comparison operation by the PLL can be easily stopped.
Further, in another example of the construction to stop the phase comparison operation by the PLL oscillation control unit, the PLL oscillation unit has an APC circuit which inputs the subcarrier and a signal based on the color burst signal and compares phases of both signals, and a loop filter connected to the APC circuit, and the PLL oscillation control unit has a switch to fix a voltage in a connection point between the APC circuit and the loop filter.
In the aspect of the present invention, the PLL oscillation unit has the APC circuit which inputs the subcarrier and the signal based on the color burst signal and compares the phases of both signals, and the loop filter connected to the APC circuit. That is, the construction is similar to the above construction where the PLL oscillation unit is constructed with the burst amplification circuit, the APC circuit and the VCO circuit. As the PLL oscillation control unit has the switch which fixes the voltage in the connection point between the APC circuit and the loop filter, the unit stops the phase comparison operation by the APC circuit by fixing the voltage using the switch. Accordingly, even if there is a phase difference between the color burst signal and the oscillation signal from the VCO circuit, the phase difference does not influence the oscillation signal, and the oscillation signal can be stabilized.
According to this aspect of the present invention, the phase comparison operation in the PLL can be easily stopped.
Note that it is the start of tuning operation that the external output unit stabilizes the reference signal to the tuner by the stoppage of phase comparison operation, and the phase comparison operation must be performed after proper tuning. As in an example of construction to exactly detect the start of tuning operation, the PLL oscillation control unit detects the start of tuning operation by detecting occurrence/non-occurrence of tuning operation by the user.
In the aspect of the present invention, the PLL oscillation control unit detects the start of tuning operation by detecting occurrence/non-occurrence of tuning operation by the user. That is, generally, the station-selection operation is started when the user has started station-selection operation to change the channel. Accordingly, the start of tuning operation is detected by the user""s station-selection operation.
According to this aspect of the present invention, the timing to stop the phase comparison operation can be easily detected.
Note that the station-selection operation can be detected by, e.g., recognizing a signal to change the channel by a remote control unit by a microcomputer in the television apparatus. In this manner, by detecting occurrence/non-occurrence of the user""s station-selection operation, the start of tuning operation can be easily detected. Further, as the user does not always perform the station-selection operation immediately after the power of the television apparatus has turned on, it is preferable to determine the period immediately after the turn-on as the start of tuning operation without any condition.
Further, when proper tuning has been made by the tuner, it is necessary to perform the phase comparison operation by the PLL oscillation unit. In a particular example for this purpose, the PLL oscillation control unit performs mode selection operation between a fixed oscillation mode to stop the phase comparison operation by the PLL oscillation unit to perform predetermined oscillation and a phase comparison oscillation mode to perform in-phase comparison operation. The PLL oscillation unit selects the fixed oscillation mode during the period from the start of tuning operation to the end of the tuning operation.
In this aspect of the present invention, the PLL oscillation control unit performs the mode selection operation between the fixed oscillation mode to stop the phase comparison operation by the PLL oscillation unit to perform the predetermined oscillation and the phase comparison oscillation mode to perform the in-phase comparison operation, and performs control to select the fixed oscillation mode during the period between the start of the tuning operation and the end of the tuning operation. As a result, the output from the external output unit is stabilized by the time proper tuning is performed. The completion of the tuning operation can be detected by using an SD signal. That is, the SD signal is outputted in accordance with judgment of non-video signal state by e.g. sampling intervals between horizontal synchronizing signals. Upon detection of the SD signal, it can be easily determined that proper tuning has been performed.
According to this aspect of the present invention, the timing to start the phase comparison operation can be easily detected.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same name or similar parts throughout the figures thereof.